This is one of two evolutionist theories regarding how reptiles—a terrestrial life form—began to fly. According to the arboreal theory, the ancestors of birds were tree-dwelling reptiles that gradually developed wings by leaping from branch to branch. (The other view is the cursorial theory, which maintains that birds took to the air directly from the ground.) But this first theory is utterly imaginary, and has no scientific evidence to support it.
John Ostrom, who first proposed the cursorial theory, admits that the proponents of both hypotheses can do nothing more than speculate. He wrote that his theory of “cursorial predator” was in fact speculative—but the arboreal theory was also speculative.36
36 John Ostrom, “Bird Flight: How Did It Begin?”, American Scientist, January-February 1979, Vol. 67.2009-08-11 15:29:41